In recent years communication systems have added many new services for consumer use. For example, a Voice Messaging System ("VMS") is used to provide a voice messaging service to a called party when the called party is unavailable. More particularly, the service of leaving a voice message for the called party is in widespread use in the case of a non-completed telephone call (e.g., where the called party is either busy or otherwise unavailable). Voice messages may in fact be more efficient than telephone conversations where participation of the two parties to a call is not required to transfer information. Voice messaging can be considered as a replacement for a human attendant who would otherwise be required to transcribe the message, or for a simple analog recording device (answering machine) that can be connected to a called telephone. In this regard see, for example, a book entitled "A Practical Guide to Voice Mail" by Martin F. Parker, Osborne McGraw-Hill, 1987 at pages 2-20 and 198-199; an article entitled "A Voice Message System" by D. Cohen in Computer Message Systems, edited by R. P. Uhlig, Proceeding of the IFIP TC-6 International Symposium On Computer Message Systems, Ottawa, Canada, 6-8 Apr. 1981, North Holland Publishing Co., at pages 17-28; an article entitled "An Experimental Voice-Message System Controlled By Word Recognition" by P. Mermelstein et al. in Computer Message Systems, edited by R. P. Uhlig, Proceeding of the IFIP TC-6 International Symposium On Computer Message Systems, Ottawa, Canada, 6-8 Apr. 1981, North Holland Publishing Co., at pages 29-34; and an article entitled "Voice Mail and Competing Services" by Dr. I. Gitman in Computer Message Systems, edited by R. P. Uhlig, Proceeding of the IFIP TC-6 International Symposium On Computer Message Systems, Washington, D.C., 5-7 Sep. 1985, North Holland Publishing Co., at pages 405-410. Limitations are found with certain VMSs when they are integrated with a Private Branch Exchange ("PBX") in a circumstance wherein a caller communicating with the VMS desires to transfer to an extension on the PBX.
Referring now to FIG. 1, there is shown a conventional system 9 comprising a Voice Messaging (or Voice Mail) System ("VMS") 10 which is integrated with (coupled to) a Private Branch Exchange ("PBX") 12 via an Integrated Voice Messaging System ("IVMS") link 14, a plurality of lines 16 comprised of Logic Terminal Numbers ("LTNs") in an Automatic Call Distribution ("ACD") Group, and a single operator extension line 18. For purposes of discussion, it is assumed that PBX 12 is a Northern Telecom SL-1 PBX. Integration of VMS 10 and PBX 12 is achieved through IVMS link 14 which passes phone call information from PBX 12 to VMS 10. The call information transferred over link 14 includes, for example, "Called Party Extension," "Calling Extension Party," and a "Reason for the Call." This call information is used by VMS 10 to provide intelligent service to a caller. In such an integrated system, the voice paths of the various callers to VMS 10 are provided from PBX 12 over lines 16 comprised of LTNs in the ACD Group.
IVMS link 14 also carries certain information from VMS 10 back to PBX 12. One use of the information passed back to PBX 12 is to have the PBX transfer a caller to a single operator extension line 18 (also called a Night Call ForWarding line or "NCFW" in a Northern Telecom SL-1 PBX). This capability is used by VMS 10 to achieve transfers to any extension on PBX 12. The following steps are used to transfer calls in the VMS/PBX integrated system of FIG. 1. In a first step, a caller engaged in a VMS session who wishes to transfer to an extension on PBX 12 enters the extension number to VMS 10. In a second step, VMS 10 notes the request for transfer and transmits a special command (e.g., an Operator (OPR) command) to PBX 12 over IVMS link 14. In a third step, PBX 12 recognizes the special command and transfers the caller to NCFW operator line 18 which then rings a call into VMS 10. In a fourth step, VMS 10 picks up the call on NCFW line 18. Since VMS 10 initiated and remembers which caller requested a transfer to a particular extension in the first step, VMS 10 performs a "Hook Flash" transfer to this extension over NCFW line 18. In a fifth and final step, when PBX 12 receives a "Hook Flash" transfer request from VMS 10 along with the extension number desired, PBX 12 rings the desired extension and connects the caller to this extension. As soon as this is accomplished, VMS 10 releases NCFW operator line 18 to handle a next transfer request.
System 9 of FIG. 1 suffers from the following limitations. First, it is difficult to accommodate a number of concurrent transfer requests which occur at a given time since only one NCFW operator line 18 is available. Due to such difficulty of accommodating concurrent transfer requests, certain VMSs restrict the transfers to only the operator or attendant. The time expended in transferring each call is objectionable to some VMS 10 users. Second, if VMS 10 is given the capability of transferring callers to any extension on PBX 12, then depending upon the number of callers desiring to transfer at the same time, some of the calls may have to wait for a long period of time in a queue before such callers finally get transferred. This occurs because a single NCFW operator line 18 can only perform one transfer at a time, and it can take up to ten seconds for each transfer. Therefore, if six callers are requesting transfers at the same time, one of the callers will have to wait up to 60 seconds before that caller can be serviced. Many VMS users object to this time delay.